Travelers make their way to the departure lounge at Sanaa International Airport, in Yemen on Tuesday.

SANAA, Yemen — A suspected U.S. drone killed four alleged al-Qaida members Tuesday in Yemen, as the U.S. and British embassies evacuated staff amid reports of a threatened attack by the terrorist group.

As Yemen bolstered security by sending tanks and troops into the streets of Sanaa, the capital of the impoverished country, militants shot down an army helicopter, killing all eight people aboard, the government said.

Yemeni authorities have suggested that there were al-Qaida threats in recent days to multiple potential targets in the country, which has been thrust back into the forefront of the international fight against the terrorist network. Among those sites were foreign installations and government offices in the capital of Sanaa as well as to the strategic Bab al-Mandeb straits at the entrance to the Red Sea in the southern Arabian Peninsula.

The State Department ordered nonessential personnel at the U.S. Embassy to leave the country. The department said in a travel warning that it had ordered the departure of non-emergency U.S. government personnel “due to the continued potential for terrorist attacks,” adding that U.S. citizens should leave immediately because of an “extremely high” security threat level.

Britain’s Foreign Office also said it has evacuated all staff from its embassy due to increased security concerns. The Foreign Office said the staff were “temporarily withdrawn to the U.K.” on Tuesday.

Yemen’s government criticized the evacuations in a statement from its embassy in Washington, saying the diplomatic withdrawal “serves the interests of the extremists and undermines the exceptional cooperation” between Yemen and the international community in the fight against terrorism. It insisted that its government has taken all precautions to ensure the security of foreign missions in Sanaa.

The U.S. has temporarily shut down 19 diplomatic posts in the Middle East and Africa after the interception of a secret message between al-Qaida chief Ayman al-Zawahri and Nasser al-Wahishi, the leader of the Yemen-based al-Qaida in the Arabian Peninsula, about plans for a major terror attack, according to a U.S. intelligence official and a Mideast diplomat. The officials spoke on condition of anonymity because they were not authorized to discuss the matter publicly.

Jan Psaki, spokeswoman for the State Department, said in a separate statement early Tuesday that the department issued the order for Yemen because of concern about a “threat stream indicating the potential for terrorist attacks against U.S. persons or facilities overseas, especially emanating from the Arabian Peninsula.”

Meanwhile, there has been a spike in apparent U.S. drone strikes against al-Qaida leaders. The attack Tuesday was the fourth in two weeks.

Yemeni officials say the drone fired a missile at a car carrying four men in the al-Arqeen district of Marib province, setting it on fire and killing them. One of the dead was believed to be Saleh Jouti, a senior al-Qaida member.

In Sanaa, residents awoke to the sound of an aircraft overhead. Officials said it was American, and photos posted on Instagram appeared to show a P-3 Orion, a manned surveillance aircraft.

The rare overflight of the capital came shortly before the announcements of the evacuations.

Yemen increased security around the presidential palace and vital state institutions. Tanks and armored vehicles were seen near the palace, and authorities set up checkpoints across the capital, searching cars and individuals, especially at night. Top government officials, along with military and security commanders, were asked to remain vigilant and limit their movements.

According to a Yemeni government official, President Abed Rabbo Mansour Hadi returned Sunday to Sanaa from the U.S., but only his son was there to meet him because of security concerns among top authorities who were told they might be targeted by al-Qaida.

The Yemeni military helicopter was shot down by a missile over the al-Qaida stronghold of Wadi Ubida in central Yemen, officials said. The helicopter was flying from Sanaa to the province of Marib, officials said. The eight who were killed, including a military commander, were part of a military force guarding oil installations in the province.

The Yemeni officials who provided the information on the suspected drone, the helicopter downing and the security in the capital all spoke on condition of anonymity because they were not authorized to talk to the media.

On Monday, Yemeni authorities released the names of 25 wanted al-Qaida suspects Monday, saying they were planning terrorist attacks in Sanaa and other cities.

The Interior Ministry said the suspects were going to target foreign offices and organizations, as well as government installations in the impoverished Arab country. It said security was increased around embassies, ports, airports, oil installations and power stations.

Officials say potential U.S. targets in Sanaa could include the embassy and other buildings used by the United States to house personnel, as well as a military facility a few miles outside used by U.S. aircraft.

The ministry statement said security forces will pay $23,000 to anyone who comes forward with information leading to the arrest of any of the wanted men. They included allegedly senior figures in al-Qaida’s Yemen branch, including Saudi nationals Ibrahim Mohammed el-Rubaish and Ibrahim Hassan el-Assiri.

El-Rubaish was released from the U.S. prison in Guantanamo Bay in 2006 and is believed to have played significant roles in al-Qaida’s expanding offshoot in Yemen. He is a theological adviser to the group and his writings and sermons are prominent in the literature.

Military officials said the threat may be related to the Bab al-Mandeb, pointing to a visit Sunday by the defense minister, Gen. Mohammed Nasser Ahmed, to Yemeni forces positioned at the Red Sea entrance about 170 miles south of Sanaa.

Officials said the visit came after they received intelligence that al-Qaida could be targeting foreign or Yemeni interests at the vital maritime corridor, a main thoroughfare for international shipping but also a crossing point for smuggled weapons and illegal immigrants between east Africa and Yemen.

Ahmed urged the forces to stay “on alert against any sabotage operations aiming at destabilizing the country,” according to the officials. They also spoke on condition of anonymity because they were not authorized to talk to the press.

Yemen has been the site of numerous anti-U.S. attacks dating back to the 2000 bombing of the USS Cole in Aden harbor, which killed 17 American sailors.

In 2008, al-Qaida attacked the U.S. Embassy in Sanaa, killing one American. Western embassies were locked up in 2010, days after the foiled plot on Christmas Day in which a passenger on a Detroit-bound plane tried to detonate explosives in his underwear.

The Obama administration alleged the suspect was trained and armed by the al-Qaida affiliate in Yemen.

The impoverished, decentralized and predominantly Muslim country is the ancestral homeland of the late al-Qaida leader, Osama bin Laden.

Washington considers the al-Qaida branch in Yemen to be among the terrorist network’s most dangerous branches. The United States has also assisted Yemen in fighting the militants who, at one point during the country’s recent political turmoil, had overrun large sections of the south. The group has also carried out bold attacks on Yemeni security forces, killing hundreds in the past two years.